This invention relates to zeolitic molecular sieve compositions characterized by outstanding capability to complex multivalent cations, especially calcium. In particular, the invention relates to novel zeolitic molecular sieve compositions, especially those based on molecular sieves having a high aluminum content, in which crystals of the zeolite are modified by the inclusion of occluded silica within the crystals.
With environmental concerns over phosphates rising during the last generation, zeolite molecular sieves have taken a dominant role as the water softening builder component of most detergents. Environmentally "friendly", zeolites have been a poor substitute for phosphates from a performance standpoint, having both lower calcium and magnesium sequestration capacities as well as much lower rates of sequestration. The sequestration properties of zeolites arise from their ability to ion-exchange. This ion-exchange ability derives from tetrahedral Al(III) inherent in classical zeolite frameworks. Each aluminum induces one negative charge on the framework which is counterbalanced by an exchangeable cationic charge. Thus, exchange capacity is limited by the aluminum content and "detergent" zeolites have been restricted to the relatively short list of "high aluminum" zeolites. By Lowenstein's Rule, the Si/Al ratio of a zeolite may not be lower than 1.0 and concomitantly, the aluminum content may not exceed 7.0 meq per gram for an anhydrous material in the sodium form. This capacity may alternatively be expressed as 197 mg CaO per gram zeolite (anhydrous) when water softening is the desired exchange reaction. Zeolites demonstrating this maximum aluminum content include Zeolite A, high aluminum analogs of Zeolite X and high aluminum analogs of gismondine (often referred to as Zeolite B, P or MAP).
While Zeolite A has been the "detergent zeolite" of choice for years, the possibility of employing a high aluminum version of gismondine-type materials in calcium sequestration has been known for more than a generation (U.S. Pat. No. 3,112,176 Haden et al.) and has recently found renewed interest (for example, U.S. Pat. No. 5,512,266 Brown, et al.). In addition to zeolites, the ability of silicates to complex ions such as calcium and especially magnesium has long been known and sodium silicate has long been employed as a cheap, low performance detergent builder. More recently, complex silicates such as Hoechst SKS-6 have been developed which are claimed to be competitive with higher performance zeolites.
The capacity for silicates to complex ions such as calcium and magnesium is inversely proportional to silicate chain length and directly proportional to the electronic charge on that chain fragment. Silicates depolymerize with increasing alkalinity (FIG. 1). At moderate pH (where wash cycles are conducted) silicates are polymeric. However, at much higher pH's silica not only become predominantly monomeric, but that monomer may possess multiple charges. If such small, highly charged fragments could be exposed to solutions bearing multivalent cations, very powerful high capacity sequestration agents would result. We believe that we have created such a situation by isolating and stabilizing substantial concentrations of such species within zeolite cages where ions such as calcium and magnesium are free to enter from an aqueous environment (such as wash water) and react with these powerful sequestration agents.
Detergent compositions based on zeolitic builders are described in U.S. Pat. No. 3,605,509 (Corkill et al.) and U.S. Pat. No. 4,663,071 (Bush et al.). The teachings of both patents are incorporated in full herein by cross-reference.